Why is Pumping Required in Concrete Placing?

Now-a-days ready mix concrete is a common and popular concrete production process. Pumping of such concrete to delivery point is very popular method of concrete placement. Here we will learn about the feasibility of pimping of concrete.

Concrete is a very viscous materials and our regular pumps have to be modified for this purpose. The economy of concrete pumping lies in uninterrupted pumping otherwise different problems arise and necessary remedial measures have to be taken which includes additional costs.

The advantages of concrete pumping are as follows:

a. In this system, we can deliver concrete over wide area.

b. Less accessible site can be covered.

c. Concrete can be supplied in special cases like tunnel lining.

d. Leave more available free space in the project site.

e. Double handling of concrete can be avoided as concrete can be delivered from mixer to required point directly.

f. The placing rate can be as that of nixing output; some time may be at rate of multiple outputs.

g. Separate transporting and subsequent placing equipment can be avoided. 

Dear reader we will discuss different aspect of economic pumping, necessary modification of concrete for this purpose, safety issues in operating concrete pump and placing will discussed in our upcoming posts; please stay with us.

 

Partial and Complete Replacement of Sand with Glass in Concrete

It is observed that a 50% replacement of sand produced peak value of workability i.e. slump of resulting concrete increased proportionally up to 50% replacement. This amount of replacement also found better for strength development. It is reported that strength of such concrete is higher than concrete produced with control mix by 10%.

In producing sustainable concrete, we are looking for less application of natural fine aggregate and introducing other filler materials like crushed glass as partial replacement, full replacement and sometimes sandless concrete. Sandless concrete is a concrete that have no fine aggregates which is replaced by cement, coarse aggregate and water; some patents are found in market, we will discuss some of these in our upcoming posts.

In case of 100% replacement of sand with glass aggregates were examined and found comparable strength with concrete produced with controlled mix after identical age of curing.

Naik and Wu have studied many aspects of sand replacement in concrete and cement replacement as well; these were mainly about partial replacement. In this application alkali-silica reaction (ASR)is the controlling factor for concrete mix design and used some ASR suppressors like

a. Cement replacement by flyash. Class F flyash are used at a percentage of 15, 30, and 45 by weight.

b. Corresponding replacements of sand with glass aggregate were also 15%, 30% and 45% of concrete sand.

Under this combination of concrete mix alkali-silica reaction, compressive strength and tensile strength (splitting) as well of resulting concrete were evaluated. The following conclusions were drawn:

a. A little reduction of compressive strength was found when sand is replaced with crushed glass.

b. A linear relationship of expansion was observed corresponding to percentage of crushed glass when flyash is absent in concrete mix, the expansion was obviously for ASR.

c. Of these different percentage of cement replacements, up to 30% replacement with flyash didn’t stop expansion, only can delay expansion i.e. no influence on long term expansion.

d. A successful suppression of ASR expansion was observed when cement replacement reached 45% irrespective of percent replacement of sand with glass aggregates. It is established that a greater than 45% cement replacement with class F flyash can control deleterious expansion.

How to Solve Alkali-Silica Reaction in Concrete Having Glass Sand Aggregates?

Now-a-days engineers are concerned with number of chemical reactions that produce effect on concrete. Of these most common is alkali silica reaction which defines chemical reaction between reactive silica in aggregates and alkalis present in cement. Here we would like to include why is this reaction harmful for understanding our present topic about application of glass as aggregate?

Under this reaction a gel of alkali-silicate product is formed which can take place in following locations:

a. Within weak planes

b. Pores in aggregates

c. Over the surface of aggregates.

The gel have swelling property and when found water, results an increase in volume thus producing expansion resulting cracks and we have learnt how much deterioration can take place under cracks in concrete.

So our topic was about replacing natural aggregates with glass. Glasses are inspecting for applying in concrete as replacement of fine aggregates, coarse aggregates and as cement replacement for many years. But most of the experiments produced unsatisfactory yields due to alkali-silica reaction as discussed above.

Now some researchers are introducing some suppressors of alkali silica reaction (ASR) to mitigate such problem and achieved some satisfactory results. An interesting fact is that remedial measure lies in glass itself i.e. if we ground glass to certain fineness it shows pozzolanic characteristics.

Glass are inspected for both coarse aggregates and fine aggregates. In case of coarse aggregates, workability of concrete is impaired badly; we have discussed this in our previous post. This is also worth mentioning that glass means waste glass as a recycled product.

Dear reader while we are discussing about solving deleterious expansion of alkali silica reaction, we have to know ASR suppressor that can be use to control such reaction; here we are listing some of these but as a general ASR suppression not considering ASR related to Glass application:

a. Fine siliceous materials

b. Application of pozzolana

c. Silica fume

d. Ground granulated blast furnace slag (GGBS)

e. Lithium salts like lithium nitrate

f. Matakaolin

Of these finely divided siliceous material, replacing cement with GGBS, fly ash, silica fume or by adding steel fibres and lithium carbonate and lithium chloride are examined. In these experiments the constituents percentage are altered to have best results. The glass aggregate that produce ASR also produce ASR suppression when ground in fine size enough to behave like pozzolana.

When glass sand (as fine aggregate) has size within (1.18 - 2.36) mm, exhibits severe alkali-silica reaction results maximum ASR expansion consequently. Here we like to include that the reaction is occurred when alkali-silica of reactive from come into contact. Now if we reduce silica content in concrete mix ASR can be controlled, at the same time if we control alkali sources ASR can also be reduced.

Glass aggregate that derived from silica-soda-lime sources produce ASR reaction from both sources i.e.

-Reactive silica

- A potential source of alkali source

Glass as a very finely divided from can also be used as cement replacement, but suitable when substitution fraction is low. There have also interesting behavior of glass that performance of glass also depends on its color, say flint and green color glass show better performance than amber glass.

Most of glass waste collected from United States are of soda-lime type; as discussed above these glasses are more susceptible to ASR reaction. In case of such application fly ash is applied to replace cement together with replacement of natural sand with crushed glass. Sometimes full replacement of sand is also tried.

Slump and Strength Increment of Glass Sand Concrete

It is observed that a progressive increment in slump of wet concrete is found with replacing sand from concrete with glass sand. The replacement is about 50% which leads to peak slump but after that a decreasing trend is observed. We can explain this; an overall better grading is achieved when equal proportion of sand and glass sand remains in fine aggregate.

For an equal time interval, the concrete produced with 50% sand replacement, shows a higher ultrasonic pulse velocity then other replacement percentage. We are trying to explain, briefly, about ultrasonic pulse velocity here.

Ultrasonic pulse velocity test for concrete is a non-destructive test. Some non destructive tests are:

a. Rebound hardness test

b. Ultrasonic pulse velocity test

c. Other types of non-destructive tests: equipments are developed for measuring different crack parameters etc.

In ultrasonic pulse velocity test, ultrasonic pulses are transmitted through desired concrete section and velocity of pulses is measured while they travel from transmitter to receiver.

There have a correlation between strength and pulse velocity. A higher velocity defines a stronger concrete relative to a slower one.

Let us know typical ranges of good pulse velocity. When pulse velocity is 4 km/s concrete is considered good. But a velocity below 3 km/s is indicative to poor concrete quality. The concrete made with 50 % replaced glass sand showed velocity about 3 km/s.

Now consider 100% replacement of sand with glass sand. In this combination of mix proportion also indicates an equal or greater compressive strength than control mixes at same time interval. A concrete mix at 28 days can achieve 49.1 Mpa compressive strength while a glass sand replaced concrete produces a slightly greater value 49 .5 Mpa.

Thus 100% replacement of fine aggregate is possible. But 50% replacement with glass sand produce 10% greater compressive strength than control mixes. As discussed in last part, this concrete has highest slump and best ultrasonic pulse velocity.

Thus it can be concluded that 50% glass replacement for sand (sand glass) is optimum value to have optimum mechanical properties from concrete made of waste glass.

Glass as Coarse Aggregate in Concrete

Different studies were conducted over glass to make it useable as concrete constituents. But previous findings and applications were limited to low strength and low density concrete which didn’t satisfied requirements of concrete engineers.

Previous studies disappointed researchers as they don’t consider followings:

a. Unsatisfactory concrete due to alkali-silicate reactions and can’t overcome this problem.

b. Selection of particles sizes for concrete constituents.

Dear reader we will discuss about second point i.e. wrong selection of particles.

There have a threshold size below which alkali-aggregate reaction is reduced and also have a threshold replacement of fine aggregate that can reduce such drawbacks. Choosing a coarser aggregate size produced concrete of poor quality. Such sizes of particles have elongated shape and smooth textures which largely contribute to poor concrete. Different types of glass produce concrete of different properties.

They can successfully be used in prepacked concrete as coarse aggregate. The intrusion mortar can penetrate fairly well and can bind glass coarse aggregate to produce a unit mass of concrete. The strength of such prepacked concrete was found adequate so as to handle and ship and transport them. Dear reader we will discuss about prepacked concrete in our upcoming posts; please stay with us.